The theory of evolution proposes that
individual life forms inherit traits from their
parents and pass those traits on to their
offspring, and that because some individuals
survive to reproduce and others do not, that the
ones that do survive will pass on more of their
traits into future generations, thus causing the
frequency of different traits in any particular
generation to change over time.

There are several different processes of
evolution concerning how these traits get passed
on and why some are passed on more than others.
The theory of natural selection states that some
traits actually help the life forms that have
them, and thus cause themselves to be passed on
more often, becoming more common in future
generations. Genetic drift is when some
individuals happen to pass on their traits and
others to not do so by sheer luck, with there
being nothing to do with how good or not good
the traits are. Mutations are the source of new
traits. All of these are subject to the
environment; for example, if climate changes,
then which traits are good for natural selection
and which traits are not so good may change in
response to climate (a thick coat of fur might
not be needed to keep you warm in a hot desert
like southern California, but it's really useful
in a colder climate).

All four of these processes have been seen at
work in nature. The unanswered questions
regarding the theory of evolution mostly have to
do with how important each of these forces is in
shaping the history of the world that we can see
in fossils, DNA sequences of living animals and
plants, etc. At this point, the theory of
evolution becomes a great deal more complicated,
and the mysteries are still numerous. Darwin,
for example, when he formulated the theory of
natural selection during the first half of the
1800s, thought that almost all extinction was
due to natural selection and specifically due to
competition between superior and inferior
traits. We don't think that's the case anymore;
the majority of extinction is more likely due to
climate change, but we still don't know for
certain.